1. Field of the Invention
The present invention relates to a semiconductor device having a circuit structured by a thin film transistor (hereafter referred to as a TFT). For example, the present invention relates to an electro-optical device, typically a liquid crystal display panel, and to electronic equipment with such electro-optical device installed as a component.
Note that, throughout this specification, semiconductor device indicates general devices which function by using semiconductor characteristics, and that electro-optical devices, semiconductor circuits, and electronic equipment are all categorized as semiconductor devices.
2. Description of the Related Art
Techniques for using semiconductor thin films (with a thickness on the order of several tens nm to several hundreds of nm) formed on a substrate having an insulating surface to structure a thin film transistor (TFT) have been in the spotlight in recent years. Thin film transistors are widely applied to electronic devices such as ICs and electro-optical devices, and their development as switching devices for image display devices is proceeding apace.
For example, the application of TFTs is being tested in all electric circuits in a liquid crystal display device: in a pixel section for controlling each of the pixels arranged in matrix; in a driver circuit for controlling the pixel section (hereafter referred to as a driver circuit); and in addition, in a logic circuit for processing external data signals (such as a processor circuit and a memory circuit).
A structure with these circuits (pixel section, driver circuits) integrated on one substrate is known (a system on panel). The pixels in the pixel region fulfill a role of maintaining information sent from the driver circuit, but unless the off current of the TFTs connected to the pixels is sufficiently low, the information cannot be stored, and a good display cannot be obtained.
In the driver circuit, on the other hand, the TFTs must have a high mobility, and the higher the mobility, the simpler the circuit structure can be made, and the faster the display device can be operated.
The characteristics required for the TFTs placed in the driver circuit and the pixel region differ, as above. Namely, the TFTs placed in the pixel region are not required to have a very high mobility, but that the off current is small and that the of f current value is uniform throughout the pixel region. On the other hand, in the TFTs for the peripherally placed driver circuits, priority on mobility is put over off current, and a high mobility is required.
However, it has been difficult to manufacture a TFT, which favors high mobility, and a low off current TFT on the same substrate without harming the reliability, and with good productivity, by using a conventional method of manufacturing.
In order to realize a system on panel with a driver circuit and a logic circuit built in, as above, a completely new, unconventional constitution is required.
The present invention answers this demand, and an object of the present invention is to provide an electro-optical device having high reliability, in which each circuit of an electro-optical device, typically AM-LCD, is formed by a TFT with a structure that appropriately responds to the circuit function.
According to a structure of the present invention disclosed in this specification, there is provided a semiconductor device comprising a driver circuit and a pixel section formed on the same substrate, characterized in that
the driver circuit and the pixel section each have an n-channel TFT made up of a channel forming region, a pair of high concentration impurity regions, and a low concentration impurity region, and in that
the concentration of a periodic table group 15 element contained in at least a portion of the n-channel TFT low concentration impurity region of the driver circuit is higher in comparison with the concentration of a periodic table group 15 element contained in at least a portion of the n-channel TFT low concentration impurity region of the pixel section.
According to another structure of the present invention, there is provided a semiconductor device comprising a driver circuit and a pixel section formed on the same substrate, characterized in that
the driver circuit and the pixel section each have an n-channel TFT made up of a channel forming region, a pair of high concentration impurity regions, and a low concentration impurity region, and in that
the concentration of a periodic table group 15 element contained in at least a portion of the n-channel TFT high concentration impurity regions of the driver circuit is higher in comparison with the concentration of a periodic table group 15 element contained in at least a portion of the n-channel TFT high concentration impurity regions of the pixel section.
According to another structure of the present invention, there is provided a semiconductor device comprising a driver circuit and a pixel section formed on the same substrate, characterized in that
the driver circuit and the pixel section each have an n-channel TFT made up of a channel forming region; a gate insulating film formed in contact with the channel forming region; a gate electrode formed in contact with the gate insulating film; a pair of low concentration impurity regions formed sandwiching the channel forming region; and a high concentration impurity region formed in contact with the low concentration impurity regions, in that
at least a portion of the n-channel TFT low concentration impurity region of the driver circuit overlaps the gate electrode through the gate insulating film, and in that
at least a portion of the n-channel TFT low concentration impurity region of the pixel section does not overlap the gate electrode.
In the above structure, the semiconductor device is characterized in that the width of the n-channel TFT low concentration impurity region of the driver circuit in the channel length direction differs from the width of the n-channel TFT low concentration impurity region of the pixel section in the channel length direction.
In the above structure, the semiconductor device is characterized in that the pixel section has an n-channel TFT made up from: a shielding layer; an insulating film formed in contact with the shielding layer; a channel forming region formed in contact with the insulating film; a gate insulating film formed in contact with the channel forming region; a gate electrode formed in contact with the gate insulating film; a pair of low concentration impurity regions formed sandwiching the channel forming region; and a high concentration impurity region formed in contact with the low concentration impurity regions.
In the above structure, the semiconductor device is characterized in that the shielding layer overlaps the channel forming region and the low concentration impurity regions through the insulating film.
In the above structure, the semiconductor device is characterized in that the width of the shielding layer in the channel length direction is wider than the width of the gate electrode in the channel length direction.
According to a structure of the present invention for realizing the above structure, there is provided a method of manufacturing a semiconductor device comprising a driver circuit and a pixel section formed on the same substrate, characterized by comprising:
a first step of forming a shielding layer;
a second step of forming an insulating film covering the shielding layer and the substrate;
a third step of forming a semiconductor layer on the insulating film;
a fourth step of performing crystallization of the semiconductor layer;
a fifth step of patterning the crystallized semiconductor layer, and of forming an active layer of the driver circuit and an active layer of the pixel section;
a sixth step of forming a gate insulating film on the active layers;
a seventh step of selectively doping a periodic table group 15 element into the active layer of the driver circuit using a first mask;
an eighth step of forming a wiring on the gate insulating film;
a ninth step of selectively doping a periodic table group 15 element into the active layers of the driver circuit and the pixel section using the wiring as a mask; and
a tenth step of selectively doping a periodic table group 15 element into the active layers of the driver circuit and the pixel section using a second mask.
In the above structure, the method of manufacturing a semiconductor device is characterized in that the concentration of the periodic table group 15 element doped in the seventh step 7 is: higher than the concentration of the periodic table group 15 element doped in the ninth step; and lower than the concentration of the periodic table group 15 element doped in the tenth step.
According to another structure of the present invention, there is provided a method of manufacturing a semiconductor device having a driver circuit and a pixel section on the same substrate, characterized by comprising:
a first step of forming a shielding layer;
a second step of forming an insulating film covering the shielding layer and the substrate;
a third step of forming a semiconductor layer on the insulating film;
a fourth step of performing crystallization of the semiconductor layer;
a fifth step of patterning the crystallized semiconductor layer, and of forming an active layer of the driver circuit and an active layer of the pixel section;
a sixth step of forming a gate insulating film on the active layers;
a seventh step of selectively doping a periodic table group 15 element into the active layer of the driver circuit using a first mask;
an eighth step of performing a first process of activating the periodic table group 15 element doped into the active layers;
a ninth step of forming a wiring on the gate insulating film;
a tenth step of selectively doping a periodic table group 15 element into the active layers of the driver circuit and the pixel section using the wiring as a mask;
an eleventh step of selectively doping a periodic table group 15 element into the active layers of the driver circuit and the pixel section using a second mask; and
a twelfth step of performing a second process of activating the periodic table group 15 element doped into the active layers.
In the above structure, the method of manufacturing a semiconductor device is characterized in that the shielding layer is formed only in the region which becomes the pixel section.
In the above structure, the method of manufacturing a semiconductor device is characterized in that the second mask is formed by back side exposure.